Macroelectronics is an emerging area of interest in the semiconductor industry. Unlike the traditional pursuit in microelectronics to build smaller devices and achieve higher degrees of integration over small areas, macroelectronics aims to construct distributed active systems that cover large areas. Often, these systems are constructed on flexible substrates with multiple types of components and allow for distributed sensing and control. A number of applications are already under consideration for macroelectronics including smart artificial skins, large-area phased-array radars, solar sails, flexible displays, electronic paper and distributed x-ray imagers. A macrofabrication technology should generally be able integrate a large number of various functional components over areas exceeding the size of a typical semiconductor wafer in a cost-effect and time-efficient fashion.
An alternative approach for construction of macroelectronic systems is to perform the integration at the device level instead of the material level. Significant infrastructure is available to cost-effectively fabricate high-performance devices on single-crystal semiconductor substrates. Even though recent advances in robotic assembly allow for positioning of up to 26,000 components per hour on plastic substrates, the relatively moderate speed, high cost, and limited positional accuracy of these systems make them unsuitable candidates for cost-effective mass production of macroelectronics.
A powerful technology that can meet all criteria for an effective macrofabrication technology is self-assembly. In a device-level integration approach based on self-assembly, functional devices are batch microfabricated and released to yield a collection of freestanding components. These components are then allowed to self-assemble onto a template, for example on a plastic substrate, to yield a functional macroelectronic system. Self-assembly, implemented in the fashion outlined above, is an inherently parallel construction method that allows for cost-effective and fast integration of a large number of functional components onto unconventional substrates. For example, it allows for integration of components made from incompatible microfabrication processes (e.g., light-emitting diodes made in compound semiconductor substrates and silicon transistors) onto non-planar flexible substrates. Key components of a self-assembly-based macroelectronic fabrication technology include: (a) development of fabrication processes that generate freestanding micron-scale functional components, (b) implementation of recognition/binding capabilities that guide the components to bind in the correct location, and (c) determination of self-assembly procedures/conditions that construct the final system with a high yield. A fluidic self-assembly method is disclosed in international application No. PCT/U.S. 2007/072038, filed Jun. 25, 2007, which is hereby incorporated by reference.
Self-assembly of micron-scale and millimeter-scale components have been studied previously both for two-dimensional (2D) and three-dimensional (3D) integration. In 2D integration via self-assembly, a template with binding sites is prepared and a collection of parts is allowed to self-assemble onto the proper binding sites. The assembly procedure is performed in a liquid medium to allow for free motion of the components. Capillary forces are used to bind the components to the template and forces resulting from fluid flow and gravity are used to move the components and drive the system toward a minimum energy state. A major drawback of demonstrated self-assembly to this date has been the requirement of post-processing. Historically, further processing of the substrate in a clean-room has been necessary to provide electrical connections and complete the assembly procedure. The need for post-processing has limited the applicability of prior-art fluidic self-assembly methods. Self-assembly has also been used for 3D integration of freestanding millimeter-scale parts or folding of components placed on ribbons into electrical circuits. In order for the full potential of these techniques to be realized, batch microfabrication processes are needed to generate a large number of micron-scale functional components that can participate in self-assembly.
The integration of micro-optical and electronic components on a common substrate has proven to be a challenging task due to the incompatibility of the respective microfabrication processes employed for the different components. Light-emitting substrates required for excitation, such as III-V materials (semiconductor alloys made from elements from Group III and Group V on the periodic table), typically require entirely different fabrication processes than CMOS- or silicon-based manufacturing processes. As a result, current integration strategies require complex fabrication techniques to achieve fully integrated devices. A simplified method for fabricating micro-optical and electronic components on a common substrate is necessary to enable future macroelectronic devices.